In contrast to conventional spin-stabilized projectiles which derive their in-flight stability from the gyroscopic forces resulting from the high rate of spin, the finned projectiles are stabilized during flight by aerodynamic forces acting on the projectile. Although projectile spin does not contribute to the stabilization of finned projectiles, a low rate of roll around the longitudinal axis is desired to minimize the adverse effects of mass and configurational asymmetries which may result from material imperfections and from manufacturing tolerances.
Fin-stabilized projectiles are ideally launched from smooth bore guns which, due to the absence of rifling, do not impart a rolling motion. Such weapons are installed, for instance, on advanced battle tanks and commonly have calibers of 60 millimeters or more.
Automatic cannons having calibers ranging approximately from 12.7 to 40 millimeters have almost exclusively rifled barrels and generally fire various types of spin-stabilized projectiles, including armor-piercing projectiles. In order to improve the armor penetration of such weapons, it is desirable to develop technology permitting successful employment from rifled gun barrels of fin-stabilized armor-piercing projectiles with their inherent high degree of terminal effectiveness. In this case, successful employment means compatibility of the ammunition with the gun and feeder system, which in turn requires the necessary structural integrity to function reliably under all operating conditions specified for such weapons while at the same time providing a projectile accuracy which is equal to or better than that of spin-stabilized projectiles fired from the same weapon.
Commonly, fin-stabilized projectiles consist of a subcaliber penetrator and a fin assembly of four or more fins attached to the rear of the penetrator. The projectile assembly is symmetric to its longitudinal axis and is fired from the gun by means of a discarding sabot. Two important functions of the discarding sabot are to support and guide the subcaliber projectile along the centerline of the gun barrel during acceleration and to form a seal to contain the propellant gasses during travel in the barrel. The latter function is accomplished by the rotating band which engages the rifling grooves of the gun barrel and in doing so imparts spin to the projectile commensurate with the rifling twist of the barrel and the projectile muzzle velocity.
Fin-stabilized projectiles reflecting the current state of the art incorporate a sliding seat between the rotating band and the sabot body. The sliding seat is designed such as to reduce by approximately 70 to 90 percent the amount of spin transmitted from the rotating band, which picks up the full spin, to the sabot body. The degree of spin transmission within the seat of the rotating band is determined by sliding friction. Thus, upon exit from the muzzle of the gun the fin-stabilized projectile has a rate of spin equal to approximately 10 to 30 percent of that of a spin-stabilized projectile launched at the same muzzle velocity.
There are two problem areas encountered with this method of firing fin-stabilized projectiles from a rifled cannon. Firstly, it is difficult to control the spin reduction in the sliding seat with a degree of repeatability necessary to assure acceptable projectile accuracy over the entire range of operating conditions specified for military employment. Variations in projectile temperature from -40.degree. to +60.degree. C., changes in humidity, finite manufacturing tolerances, contamination by dust, salt and other substances entering between the rotating band and its seat, etc., influence the friction coefficient in the band seat and with it the degree of spin transmission.
Secondly, centrifugal forces acting on sabot components are very effective in initiating the instantaneous and symmetric separation of the sabot from the penetrator upon exit from the muzzle of the gun. With reduced projectile spin the centrifugal forces acting on the sabot components are reduced by the square of the spin ratio. As a result, the sabot separation is neither as rapid nor as precise as with a nonslipping rotating band and is increasingly more dependent on aerodynamic forces.
The access of aerodynamic forces to the projectile is delayed by the efflux of high velocity propellant gasses upon exit of the projectile from the muzzle of the gun. These propellant gasses envelop the projectile temporarily in a reverse flow field. Only upon entering into the ambient air, which occurs at a range of approximately 30 calibers from the muzzle, do the aerodynamic forces become fully effective in sabot separation. The magnitude of the aerodynamic forces prevailing for sabot separation is only a fraction of the centrifugal forces available when launching at full spin and therefore a considerably more fragile sabot construction is required to assure its fracture and separation. In addition, because of size limitations of ammunition of calibers up to 40 millimeters, the physical dimensions of sliding rotating bands, inclusive of their seats, are small, thus resulting in rather delicate and vulnerable components. In contrast, utilization of a nonslipping rotating band allows for the use of a stronger sabot which is advantageous when employed from high rate of fire cannons and their correspondingly high structural loads during feeding and ramming.
Fin-stabilized projectiles equipped with discarding sabots incorporating slipping rotating bands experience considerable variations in spin rate at exit from the muzzle due to deviations in the friction coefficient within the sliding seat of the band. As a result the subsequent acceleration or deceleration of the projectile spin may result in conditions where the spin rate is equal to the nutation frequency of the projectile and resonance instability will occur. The lower projectile spin rate at muzzle exit and consequent reduction in centrifugal forces acting on the sabot decrease the rapidity and symmetry of the discard of the sabot components and therewith result in increased projectile dispersion.
In summary, the shortcomings encountered with discarding sabot fin-stabilized projectiles for automatic guns having spin reducing sliding rotating bands are:
1. Considerable variations in projectile spin at launch due to deviations of the friction coefficient in the sliding rotating band seat.
2. Fluctuations in projectile spin at launch which results in reduced repeatability of sabot separation and subsequent projectile trajectory, thus increasing projectile dispersion and degrading first round hit probability.
3. Reduction of projectile spin at launch which decreases the centrifugal forces desired for sabot separation, thus demanding a weaker sabot construction. The loss of ruggedness and reliability is further aggravated by the vulnerability of the sliding rotating band and its seat.
Proponents of the use of sliding rotating bands erroneously assume the existence of high aerodynamic drag forces during aerodynamic despinning of projectiles having full spin rate at launch. Firing tests have demonstrated that such induced drag is minimal at most which is not surprising considering that the rotational energy of typical subcaliber fin-stabilized projectiles is less than one percent of their translatory kinetic energy. In this connection it is also of interest that because of the precise and symmetric sabot separation of the fully spun up fin-stabilized projectile, the maximum projectile yaw measured at launch was found to be less than five degrees. This low level of initial yaw is highly desirable to minimize aerodynamic drag and projectile retardation.